The present invention relates to a valve for a fuel system for a combustion engine and also to a method for controlling a fuel system for a combustion engine.
One way of reducing discharges of emissions from diesel engines is to inject fuel at high pressure into the combustion chamber, usually involving a so-called common-rail system. A common-rail system comprises a high-pressure pump which pumps fuel at high pressure to an accumulator tank situated between the pump and the engine's cylinders. The fuel in the tank is intended to be distributed to all of the cylinders. The fuel from the tank is injected into the combustion chambers of the respective cylinders by electrically operated fuel injectors. Fuel injectors have to be able to open and close very quickly and are operated by an electrical control unit which substantially continuously calculates the amount of fuel to be supplied to the respective cylinders on the basis of information from various engine parameters, inter alia engine load and engine speed.
A fuel system as above comprises valves provided with a sealing element in the form of a ball, a ball retainer and a seat. The ball may for example be made of ceramic material with a view to being wear-resistant. However, ceramic materials are brittle. Mishandling or such occurrences as a hard extraneous particle coming into contact with the ball when the valve is about to close may result in the ball disintegrating into a plurality of small fragments and being flushed away, leading to the valve no longer sealing.
If the valve serves as a safety valve in the fuel system and no longer seals, fuel will be led back to the fuel tank. If said valve is situated between the high-pressure pump and the fuel injectors, this means that the injectors will not be supplied with fuel in the same amount as previously, leading to the engine producing a lower power output or not receiving enough fuel to be able to operate.
Moreover, if the valve is in a fuel injector, the spring preloading which acts upon a valve retainer will decrease because of reduced preloading, also causing the valve not to be able to counteract such a large pressure as when the ball was in the valve and sealing against the seat. This leads to the valve no longer sealing, with the result that it is no longer possible to control the dosing of fuel into the engine's cylinders.
Uncontrolled fuel injection leads to overdosing of fuel in the cylinder, which means that the exhaust system will receive a large amount of unburnt fuel, with the potential consequence that the exhaust post-treatment system may be damaged and its service life be considerably reduced. When the engine is under heavy load, the fuel in the exhaust post-treatment system may also ignite, causing therein an undesirable temperature rise and thereby further damaging the exhaust post-treatment system and reducing its service life. The fuel consumption and the emissions in the exhaust gases will also increase.
There are various possible ways of detecting uncontrolled fuel injection, e.g. the engine's control unit may measure the exhaust temperature after an oxidation catalyst in the exhaust post-treatment system. If this temperature increases unreasonably quickly, it is very likely that uncontrolled injection has occurred. The engine's control unit may also measure the acceleration of the flywheel at each cylinder ignition. If any cylinder gives the flywheel a deviating acceleration, this too may indicate uncontrolled fuel injection. The engine's control unit controls the flow from the high-pressure pump to the fuel injectors. If the engine's control receives a signal from the pump to increase the flow abnormally sharply, this may likewise mean that uncontrolled fuel injection has occurred.
The measure currently employed to prevent reduced service life and damage to the exhaust post-treatment system when uncontrolled injection has been detected is to throttle the fuel supply to the fuel injectors, leading to the engine being switched off automatically. Alternatively, the driver is warned that a valve failure has occurred, leading to the driver switching off the engine. This is a major disadvantage in making it impossible to use the engine to run the vehicle to a workshop for remedial action.
An example of a known fuel injector and its function is referred to in SE-C2-529810. The injection means comprises a passage for receiving fuel at a first positive pressure, a pressure amplifier and an injection valve which can be put into an open position and a closed position. The injection valve comprises a piston element, a closing chamber and an opening chamber. When fuel is intended to be injected, the pressure amplifier generates a higher second fuel pressure in the opening chamber so that the piston element is moved towards an open position and fuel is injected. When the injection of fuel is to be halted, the high pressure in the opening chamber is reduced and the closing chamber is connected to said fuel source at the first pressure so that the valve is moved towards the closing position by a force provided jointly by the first pressure and said spring means.
Against the background of the above deficiencies of known valves, there is a need for a novel valve for fuel systems.